1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
It has been proposed to use as an array of individually controllable elements to pattern a beam of radiation a matrix addressable surface having a viscoelastic (e.g., having viscous as well as elastic properties) control layer and a reflective surface. When the viscoelastic control layer is addressed, its surface deforms to form, for example, a sinusoid. The basic principle behind such an apparatus is that addressed areas of the reflective surface reflect incident light as diffracted light because the sinusoidal shape of the reflective surface acts as a grating, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate spatial filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light to reach the substrate. In this manner the beam becomes patterned according to the addressing pattern of the matrix addressable surface.
A corresponding device has also been proposed using an array of diffractive optical MEMS devices. Each diffractive optical MEMS device is comprised of a plurality of reflective ribbons that can be deformed relative to one another to form a grating that reflects incident light as diffracted light.
However, arrangements for the array of individually controllable elements as discussed above are difficult to manufacture. In particular, the formation of the control circuitry below the reflective surfaces imposes constraints on the order of the processing steps during manufacture. Furthermore, the individual elements typically require a significant amount of space around them. This is especially true for diffractive optical MEMS devices. For example, this can be done to provide the drive electronics. This prevents dense packing of the individually controllable elements.
A further alternative used as the array of individually controllable elements is a matrix arrangement of small mirrors. The mirrors are matrix addressable, such that each mirror can be independently controlled to reflect incoming radiation in a desired direction. Only radiation reflected in a given direction is projected onto the substrate (i.e., which enters the pupil of the projection system). Accordingly, by appropriate addressing of the individual mirrors, the radiation beam can be patterned as required. However, it is difficult to execute in practice because the position of each mirror must be very precisely controlled.
Thus, what is needed is a system and method for use in a lithographic apparatus with an improved array of individually controllable elements.